1. Field of the Invention
The present invention relates to a semiconductor device, more particularly, to a semiconductor device having a fuse element which blows by laser irradiation.
2. Description of the Related Art
Fuse elements are generally and widely used in adjustment of a resistance value of a semiconductor device or fixing a setting of a redundant circuit. Cutting of a fuse element gives a change from a conductive state to a non-conductive state occurs, and desired information is stored in a trimming circuit. Cutting of a fuse element is done by a method of making a fuse element blow by laser irradiation, a method of making a fuse element melt and separate by a flow of a large current, or the like.
A fuse element which blows by laser irradiation is cut by an irradiation of laser light through an insulating film of silicon oxide or the like which covers the fuse element formed of a conductive body such as polycrystalline silicon (see, for example, Japanese Patent Application Laid-open No. 2000-40388).
In recent years, along with downsizing of semiconductor devices, downsizing of fuse elements is also demanded. Space between adjacent fuse elements, in particular, needs to be narrowed when a plurality of fuse elements is arranged in parallel.
FIG. 7A and FIG. 7B are diagrams for illustrating the configuration of a downsized semiconductor device 400 of the related art. FIG. 7A is a plan view of an area of the semiconductor device 400 that has a plurality of fuse elements formed therein. FIG. 7B is a sectional view taken along the line B-B of FIG. 7A.
As illustrated in FIG. 7A and FIG. 7B, the semiconductor device 400 of the related art has, on an insulating film 42 formed on a semiconductor substrate 41, a plurality of fuse elements 43 (43a, 43b, and 43c) arranged close to one another and formed of a conductive body such as a polysilicon film. An insulating film 45, which covers the plurality of fuse elements 43, is further provided, and a fuse opening 46 for laser irradiation is formed in the insulating film 45.
FIG. 8A, FIG. 8B, and FIG. 8C are diagrams for illustrating an example of problems that occur during cutting of the fuse elements 43 in the semiconductor device 400 of FIG. 7A and FIG. 7B. FIG. 8A is a plan view corresponding to FIG. 7A. FIG. 8B is a sectional view taken along the line B-B of FIG. 8A. FIG. 8C is a sectional view taken along the line C-C of FIG. 8A.
Of the three fuse elements 43 illustrated in FIG. 8A, FIG. 8B, and FIG. 8C, the fuse element 43a on the left side and the fuse element 43b in the middle are cut.
As illustrated in FIG. 8A, FIG. 8B, and FIG. 8C, when the fuse elements 43a and 43b arranged close to each other are each irradiated by laser light, portions of the conductive body of the fuse elements 43a and 43b, which are irradiated by laser light, are melted and evaporated, thereby raising the vapor pressure and exploding along with portions of the insulating film 45 covering the irradiated portions. The fuse elements 43a and 43b consequently fall into a non-conductive state.
However, the narrow space between the adjacent fuse elements 43 causes a fuse blow mark 47, which is formed as a result of the laser irradiation, to be continuous with its adjacent fuse blow mark 47.
The melted and evaporated conductive body may not blow off far enough and may readhere to inner side surfaces of the fuse blow marks 47 to form a readhesion layer 48 accidentally as illustrated in FIG. 8A and FIG. 8C. Specifically, the readhesion of the evaporated conductive body causes an electrical short circuit between the adjacent fuse elements 43 that have been cut.
This problem occurs for the following reason.
The semiconductor device 400 uses as the insulating film 45 a BPSG film or a PSB film, which is highly moisture-resistant, in order to prevent moisture seeping from the outside from corroding the fuse elements 43, wiring (not shown), and other components. However, while having excellent moisture resistance, a BPSG film and a PSG film are low in mechanical strength.
When the insulating film 45 is low in mechanical strength, the conductive body irradiated with laser light explodes at a point where the vapor pressure of the conductive body is not high enough. The conductive body consequently is not blown far, and chances are high that the melted and evaporated conductive body readheres to the inside of the fuse blow mark 47. With the space between the adjacent fuse elements 43 narrowed for size reduction, which causes the adjacent fuse flow marks 47 to be continuous with each other as described above, the readhesion layer 48 is formed in a pattern that connects the cut fuse elements 43a and 43b as illustrated in FIG. 8A. The fuse elements 43a and 43b are short-circuited as a result.
FIG. 9A and FIG. 9B are diagrams for illustrating another example of the problems that occur during cutting of the fuse elements 43. FIG. 9A is a plan view corresponding to FIG. 7A. FIG. 9B is a sectional view taken along the line B-B of FIG. 9A.
Of the three fuse elements 43 illustrated in FIG. 9A and FIG. 9B, the fuse element 43b in the middle is cut.
When the fuse element 43b is blown by being irradiated with laser light, its adjacent fuse element 43 (the fuse element 43c in this example) may partially be exposed inside the fuse blow mark 47 as illustrated in FIG. 9A and FIG. 9B.
Specifically, the low mechanical strength of the insulating film 45 described above permits the fuse blow mark 47 to spread to an area over the fuse element 43c, which is adjacent to the fuse element 43b, and an exposed portion EXP is formed in the fuse element 43c. The exposure of the fuse element 43c leads to such problems as the corrosion of the fuse element 43c from moisture and disconnection of the fuse element 43c due to oxidation started at the exposed portion EXP.
The frequency of problems illustrated in FIG. 8A to FIG. 8C and FIG. 9A and FIG. 9B increases particularly when the space between adjacent fuse elements is 5 μm or less.